Magnesium alloy for cast iron



United States Patent 3,336,118 MAGNESIUM ALLOY FOR CAST IRON George B.Newitt, Wheaton, IH., assignor to Alloy Metal Products, Inc., Davenport,Iowa, a corporation of Iowa No Drawing. Filed Nov. 9, 1964, Ser. No.410,001 4 Claims. (Cl. 29-192) ABSTRACT OF DISCLOSURE An improved ironnodularization alloy containing maximally 20% silicon, minimally 15%magnesium, minimally 50% nickel, and the balance iron. Small percentagesof the rare earth, cerium, may be added to re place some of the iron asan additional inoculant. As an article of manufacture, the briquetteform of the alloy is set forth.

This invention relates to a new alloy useful as an addition agent forthe treatment of molten iron prior to casting and to a process fortreating cast iron to produce ferrous products with improved propertiesand characteristics. In particular the invention relates to an alloywhich when added to molten cast iron will render the graphite in theiron spheroidal.

It is well known in the art of making cast iron that carbon present asgraphite, may be in one of two formsflake or spheroidal. In flake form,the graphite provides weakening discontinuities in the iron crystallinestructure resulting in a cast iron of greatly reduced tensile strength,fatigue resistance, toughness and ductility. When graphite is inspheroidal form, however, a more regular iron crystalline structureresults eliminating the deleterious effects vf flake graphite.

It is further well known that inoculation of molten cast iron withsilicon enhances graphitizing of carbon present in the iron, i.e.prevents carbon from reacting with iron to form carbides which renderthe iron hard, brittle and difficult to work. Further it is well knownthat by proper inoculation shrinkage of an iron melt during coolingafter casting is minimized.

It is well known that spheroidal graphite is produced in iron by addingmagnesium to an iron melt. However, attempts to add magnesium to molteniron results in violent explosions since the boiling point of magnesiumis below the temperature of iron melts. In order to successfullyintroduce magnesium into molten iron, many attempts have been made toalloy magnesium with other materials so that upon addition to molteniron no exp1osive vaporization occurs. U.S. Patent 2,675,308 sets out acomprehensive summary of attempts in the art to use magnesium as anaddition agent to produce spheroidal graphite in cast iron. Further,U.S. Patent 2,563,859 describes a special alloy of nickel, magnesium andsilicon which when added to molten iron results in a cast iron in whichgraphite present is in compacted or spheroidal form. The alloy describedcontains from about 32% to about 50% silicon, about 12% to 20%magnesium, up to about 12% iron, and the balance essentially nickel.

Although such alloy does produce some spheroidal graphite and eliminatesthe danger of adding elemental magnesium to molten iron baths, therestill exists the problem of shrinkage of the batch when molded andcooled and further the production of even more spheroidal graphiterendering the iron accordingly stronger.

It has now been discovered that if there is employed as an inoculationmaterial an alloy of silicon content maximally 20%, nickel contentminimally 50%, iron content minimally 15% and the balance magnesium,there is produced cast iron containing residual magnesium of .02% to.05% with a microstructure showing a high quantity of spheroidalgraphite, and which exhibits little or no 3,336,118 Patented Aug. 15,1967 shrinkage while cooling. It is essential that silicon not bepresent in the alloy more than 20%. In addition, it has been discoveredthat when employing such alloy, graphite rendered spheroidal by themagnesium remains spheroidal for a longer time while the iron is moltenthus permitting a greater length of time to utilize iron in its moltenstate. Thus, use of my new alloy permits a foundry to use a heavier postinoculation of ferrosilicon which aids in preventing the fading reactionwherein spheroidal graphite reverses to graphite flake.

It has further been found that when using the new alloy material, theresultant cast iron shows high tensile strength, yield point and goodelongation properties. Such addition agent further minimizes the amountof inclusions usually found in cast iron and thus improves the matrix ofthe iron and results in a more uniform distribution of graphite. Such isbelieved due to the thermodynamics of the oxidation-reduction reactionsoccurring between iron, carbon, silicon and magnesium.

As stated above, my new alloy permits greater postinoculation offerrosilicon, i.e. having an alloy with low silicon content means thatfoundry operators can increase the percentage of after-addedferrosilicon, or post-inoculation, and still insure that the finalsilicon percentage in the ductile iron castings is in a predetermineddesired range. Increasing the percentage of late ferrosilicon addi tion,as opposed to adding large percent-ages of silicon as part of the alloy,greatly improves the graphite dis-tribution, results in better formedspheroids, disperses and reduces carbides to a minimum, reduces internaland external shrinkage, reduces scrap due to cracking of castings, cutscosts of heat treatment due to less carbide formation in the matrix ofthe ductile iron, aids in the proper pattern shrinkage, decreases thetime for fading to start, improves machineability of castings and showsa marked improvement in the tensile, yield and elongation strengths ofcast products.

The new alloy addition agent thus provides the proper combination ofsilicon, magnesium, iron and nickel which greatly aids in propelling thethermodynamics of the interstitial reaction which occur and which areresponsible for the formation and orientation of spheroidal graphite.This results in a preferred orientation of graphite in the iron.

It is thus an object of this invention to provide a new alloy additionagent comprising a magnesium containing material of specific elements inspecific proportions, particularly adapted to the treatment of cast ironbaths to improve the casting produced therefrom.

Another object of the invention is to provide an improved process forproducing cast iron containing spheroi dal graphite which utilizes mynew alloy addition agents.

A further object is to provide an inoculation alloy low in siliconcontent thus producing a cast iron to which high post-inoculation ofsilicon or ferrosilicon can be elfected without changing the final andoverall silicon content.

In particular, the present invention contemplates a magnesium-containingalloy comprising 50% nickel, 20% silicon, 15% magnesium, and 15% iron.Alternatively, the alloy may contain 0.1% to 5% cerium or other rareearth metals. In the latter case, the rare earth metal replaces part ofthe iron in the alloy.

When employing the :above addition agent, magnesium may be added tomolten cast iron baths without danger of explosive reaction. The castiron resulting from addition of such alloy contains magnesium from .02%to .05% which is known in the art as a proper quantity for producinggraphite in spheroidal form.

The additional alloy of the invention may be compounded in any mannerknown in the art e.g. by melting, or by pelletizing or briquettingpowdered metals. The

resultant lumps, granules, pellets or briquettes may be introduced intothe ladle of molten iron in varying amounts as required by good foundrypractice. The alloy may either be placed in the ladle and the molteniron poured into the ladle or the material may be used in a plungingdevice which is immersed into a ladle of molten iron.

Magnesium retained in cast iron, as is known in the art, has been foundto have a decided effect in whitening or carbide stabilizing which isundesirable in gray iron castings. Accordingly, in the production ofmagnesium containing gray cast iron, i.e. cast iron whereinsubstantially no carbodization has occurred it has usually beennecessary to inoculate magnesium-containing cast iron baths beforecasting with a graphitizing material such as silicon or ferrosilicon andto cast the baths in an inoculated condition. It is a feature of thepresent invention that magnesium introduction and initial siliconinoculation are done simultaneously. This is effected by having siliconincluded in the alloy material.

Such simultaneous inoculation has two effects. First, the siliconpresent in the alloy in maximally 20% insures graphitization of carbonpresent in the iron. Second, the magnesium renders the graphitespheroidal in form. Further, by using initially no more than 20% siliconin the alloy, it has been found that the graphite remains in spheroidalfor-m for longer periods while the iron is molten thus permitting agreater post-inoculation of silicon if such is desired.

In a preferred embodiment of the invention, cerium is used in the alloyin 0.1% to 5% by weight. Cerium as well as other rare earth metals, is awell known gettering agent which masks the harmful effects of trampimpurities such as tin or lead. Such impurities, unless masked, causecracking of the iron and adversely affect welding properties.

In order to more fully understand the invention the followingillustrated examples are given.

Example I A cast iron melt was established and the temperature of thebath was adjusted to about 2700 F. to permit successful casting. To thebath was added 1% of an alloy containing magnesium, silicon, 15% ironand 50% nickel. The molten metal was then cast. On cooling, the castshowed substantially no center line shrinkage. The resultant castingcontained 0.03% magnesium, and was found to have excellent tensilestrength, yield point, and elongation properties. A photomicrograph of apolished surface of the iron showed the graphite to be present insubstantially all spheroidal form.

Example II A molten iron bath was prepared as in the Example I. To thebath was added an alloy, being 1% of the molten bath, containing nickel,20% silicon, 15% magnesium, 14% iron, and 1% cerium. The molten metalwas cast. Substantially no center line shrinkage was observed incooling. The castings were found to contain about .035 magnesium, .andhad excellent tensile strength, yield point, and elongation properties.

It is to be understood as hereinbefore stated that the silicon contentof the alloy addition agent of this invention must contain at maximum20% silicon and at minimum 50% nickel. Iron content must not be below15% unless cerium is used in which event the iron content may be as lowas 10%.

I claim:

1. A composition of matter useful as an addition agent for the treatmentof cast iron baths to improve the quality of castings made therefrom,consisting essentially of an alloy of 15% magnesium, 20% silicon, 50%nickel and the balance iron.

2. A composition of matter useful as an addition agent for the treatmentof cast iron baths to improve the quality of castings made therefrom, asset forth in claim 1 including 0.1% to 5% cerium.

3. As an article of manufacture, a briquette of powdered metalsconsisting essentially of 15% magnesium, 20% silicon, 50% nickel and thebalance iron.

4. As an article of manufacture, a briquette of powdered metals as setforth in claim 3 including 0.1% to 5% cerium.

References Cited UNITED STATES PATENTS 2,675,308 4/1954 Millis et a1.-130 2,690,392 9/1954 Millis et a l. 75130 3,030,205 4/1962 Millis eta1. 75-130 DAVID L. RECK, Primary Examiner.

PAUL WEINSTEIN, Assistant Examiner.

3. AS AN ARTICLE OF MANUFACTURE, A BRIQUETTE OF POWDERED METALSCONSISTING ESSENTIALLY OF 15% MAGNESIUM, 20% SILICON, 50% NICKEL AND THEBALANCE IRON.